AXI) THE F(^K'rAL MEMIiKAXES, IN THE OHICK'. ^.\\ 



such ;is lias been übser\efl in Chelonia. Hut, still, we see the siinilaritv 

 betwo^en them in the pî-ocess of folding. From the tiinc when the 

 amniotic fold ha« C(jme to enclose some of tfie anterior part of the head, 

 the epiblast seems to grow backwards always just a little in advance of 

 the mesoblast, forming at the poslerioi- edge a mass of epiblastic cells 

 in the median line. Thus in Fi^-. 1 and ¥"i<r, {^) we see that a mass 

 of tlie epiblastic cells is inserted between the two sheets of the meso- 

 blastic septum. The same state is also o13ser^'ed in Chelonia when the 

 amniotic fold has advanced considerably over the d<jrsa] part of tlie 

 emljryo. Conse(|uently the chick differs from Chelonia only by the 

 fict that the early or solely epiblastic stage of the folding is abbreviat- 

 ed, — and with res])ect to this point it is highly interesting to notice thai 

 in the chick the anmiotic fold is raised much later than in Chelonia. 



Tlie wedge-shaped e])iblastic cell-mass near the posterior edge, 

 inserted between the mesoblastic sheets, directly connects the ej)iblastic 

 layer of the amnion and that of the serous envelope, and thus is 

 seen at this stage quite the inci[»ient condition of the sero-amniotie 

 connection. Finally, the sero-aniniotic connection lies in the direct 

 prolongation of the line ol' tlie mesoblastic septum formed in 

 front of it. 



^. I'rulofKjaliun of the Serd-^iiiiitiutlc CoiuucdDn. 



From tlie time when the first trace of the sero-aniniotic connec- 

 tion has appeared, the wedge-shaped mass of the epiblastic <'ells in the 

 posterior edge of the head fold (referred to in the previous section) is 

 enormously thickened in tlie dorso-ventral direction especially ne;ir 

 tiu; median line. From this place the mass is gradually attlenuated 

 posiei'o-latei'ally along the horse-shoe sha[»ed edge (Figs. 1— o, 8<S— 4(1 

 and ()2-()5). Conse(juently along the posterior edge of the amniotic 

 fold there is t"orme(l a delta-sliaped epiblastic mass, which marks the 



